Several developmental and morphogenetic factors govern diversity of stomatal patterning in land plants

Weevaluate stomatal development in terms of its primary morphogenetic factors and place it in a phylogenetic context, including clarification of the contrasting specialist terms that are used by
different sets of researchers. The genetic and structural bases for stomatal development are well conserved and increasingly well understood in extant taxa, but many phylogenetically crucial
plant lineages are known only from fossils, in which it is problematic to infer development. For example, specialized lateral subsidiary cells that occur adjacent to the guard cells in some taxa can
be derived either from the same cell lineage as the guard cells or from an adjacent cell file. A potentially key factor in land-plant evolution is the presence (mesogenous type) or absence
(perigenous type) of at least one asymmetric division in the cell lineage leading to the guardmother cell. However, the question whether perigenous or mesogenous development is
ancestral in land plants cannot yet be answered definitively based on existing data. Establishment of ‘fossil fingerprints’ as developmental markers is critical for understanding the evolution of
stomatal patterning. Long cell–short cell alternation in the developing leaf epidermis indicates that the stomata are derived from an asymmetric mitosis. Other potential developmental
markers include nonrandom stomatal orientation and a range of variation in relative sizes of epidermal cells. Records of occasional giant stomata in fossil bennettites could indicate
development of a similar type to early-divergent angiosperms.